2015-12-07 20:16:25 +00:00
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.. index:: contract, state variable, function, event, struct, enum, function;modifier
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2016-02-18 10:45:15 +00:00
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.. _contract_structure:
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2015-12-07 20:16:25 +00:00
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***********************
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Structure of a Contract
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***********************
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Contracts in Solidity are similar to classes in object-oriented languages.
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2016-02-19 10:47:16 +00:00
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Each contract can contain declarations of :ref:`structure-state-variables`, :ref:`structure-functions`,
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2017-12-18 12:31:40 +00:00
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:ref:`structure-function-modifiers`, :ref:`structure-events`, :ref:`structure-struct-types` and :ref:`structure-enum-types`.
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2015-12-07 20:16:25 +00:00
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Furthermore, contracts can inherit from other contracts.
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2016-02-19 10:47:16 +00:00
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.. _structure-state-variables:
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2016-02-18 10:45:15 +00:00
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State Variables
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===============
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State variables are values which are permanently stored in contract storage.
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::
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2017-10-29 13:28:42 +00:00
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pragma solidity ^0.4.0;
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2016-09-05 11:54:54 +00:00
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2017-10-29 13:28:42 +00:00
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contract SimpleStorage {
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uint storedData; // State variable
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// ...
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}
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2016-02-18 10:45:15 +00:00
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2016-02-19 10:30:00 +00:00
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See the :ref:`types` section for valid state variable types and
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2017-02-02 23:52:34 +00:00
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:ref:`visibility-and-getters` for possible choices for
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2016-03-01 11:15:20 +00:00
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visibility.
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2016-02-19 10:30:00 +00:00
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2016-02-19 10:47:16 +00:00
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.. _structure-functions:
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2016-02-18 10:45:15 +00:00
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Functions
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=========
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Functions are the executable units of code within a contract.
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::
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2017-10-29 13:28:42 +00:00
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pragma solidity ^0.4.0;
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2016-09-05 11:54:54 +00:00
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2017-10-29 13:28:42 +00:00
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contract SimpleAuction {
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2017-12-12 18:47:30 +00:00
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function bid() public payable { // Function
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2017-10-29 13:28:42 +00:00
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// ...
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}
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}
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2016-02-18 10:45:15 +00:00
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2016-02-19 10:47:16 +00:00
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:ref:`function-calls` can happen internally or externally
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2017-02-02 23:52:34 +00:00
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and have different levels of visibility (:ref:`visibility-and-getters`)
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2016-05-13 14:32:35 +00:00
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towards other contracts.
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2016-02-19 10:30:00 +00:00
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2016-02-19 10:47:16 +00:00
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.. _structure-function-modifiers:
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2016-02-18 10:45:15 +00:00
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Function Modifiers
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==================
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2016-02-19 11:05:56 +00:00
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Function modifiers can be used to amend the semantics of functions in a declarative way
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(see :ref:`modifiers` in contracts section).
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2016-02-18 10:45:15 +00:00
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::
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2016-05-13 14:32:35 +00:00
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2018-04-15 21:12:28 +00:00
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pragma solidity ^0.4.22;
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2016-09-05 11:54:54 +00:00
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2017-10-29 13:28:42 +00:00
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contract Purchase {
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address public seller;
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2016-05-13 14:32:35 +00:00
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2017-10-29 13:28:42 +00:00
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modifier onlySeller() { // Modifier
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2018-01-03 14:30:01 +00:00
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require(
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msg.sender == seller,
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"Only seller can call this."
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);
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2017-10-29 13:28:42 +00:00
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_;
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}
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2016-05-13 14:32:35 +00:00
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2018-08-09 13:36:00 +00:00
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function abort() public view onlySeller { // Modifier usage
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2017-10-29 13:28:42 +00:00
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// ...
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}
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}
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2016-02-18 10:45:15 +00:00
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2016-02-19 10:47:16 +00:00
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.. _structure-events:
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2016-02-18 10:45:15 +00:00
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Events
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======
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Events are convenience interfaces with the EVM logging facilities.
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::
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2018-03-02 11:29:48 +00:00
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pragma solidity ^0.4.21;
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2016-09-05 11:54:54 +00:00
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2017-10-29 13:28:42 +00:00
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contract SimpleAuction {
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event HighestBidIncreased(address bidder, uint amount); // Event
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2016-05-13 14:32:35 +00:00
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2017-12-12 18:47:30 +00:00
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function bid() public payable {
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2017-10-29 13:28:42 +00:00
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// ...
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2018-02-16 16:32:30 +00:00
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emit HighestBidIncreased(msg.sender, msg.value); // Triggering event
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2017-10-29 13:28:42 +00:00
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}
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}
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2016-02-18 10:45:15 +00:00
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2016-05-13 14:32:35 +00:00
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See :ref:`events` in contracts section for information on how events are declared
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2016-02-19 11:05:56 +00:00
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and can be used from within a dapp.
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2017-12-18 12:31:40 +00:00
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.. _structure-struct-types:
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2016-02-18 10:45:15 +00:00
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2017-12-18 12:31:40 +00:00
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Struct Types
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2016-02-18 10:45:15 +00:00
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=============
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2016-05-13 14:32:35 +00:00
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Structs are custom defined types that can group several variables (see
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2016-02-19 11:05:56 +00:00
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:ref:`structs` in types section).
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2016-02-18 10:45:15 +00:00
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::
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2017-10-29 13:28:42 +00:00
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pragma solidity ^0.4.0;
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2016-09-05 11:54:54 +00:00
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2017-10-29 13:28:42 +00:00
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contract Ballot {
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struct Voter { // Struct
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uint weight;
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bool voted;
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address delegate;
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uint vote;
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}
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}
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2016-02-18 10:45:15 +00:00
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2016-02-19 10:47:16 +00:00
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.. _structure-enum-types:
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2016-02-18 10:45:15 +00:00
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Enum Types
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==========
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2018-04-25 17:32:50 +00:00
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Enums can be used to create custom types with a finite set of 'constant values' (see
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2016-02-19 11:05:56 +00:00
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:ref:`enums` in types section).
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2016-02-18 10:45:15 +00:00
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::
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2016-05-13 14:32:35 +00:00
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2017-10-29 13:28:42 +00:00
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pragma solidity ^0.4.0;
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2016-09-05 11:54:54 +00:00
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2017-10-29 13:28:42 +00:00
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contract Purchase {
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enum State { Created, Locked, Inactive } // Enum
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}
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